In radio frequency (RF) transmission systems, the so-called maximum power transfer theorem states that the maximum amount of power can be transferred to a load resistance when that load resistance is equal to the source resistance of the circuit supplying the power. In the case of an amplifier driving a transmission line, it is important for good power transfer to closely match the output impedance of the amplifier to the impedance of the transmission line. In addition the matching network can suppress harmonics of the carrier frequency to comply with regulatory levels. In known systems, the termination impedance of a transmitter power amplifier can be matched to the transmission line using LC ladder networks. In addition to matching transmission line impedance, LC ladder networks also can suppress unwanted harmonics and help meet governmental regulations for harmonic suppression.
One known LC ladder network is a simple two-element discrete LC ladder network that can be used for lower-power designs. The two-element LC ladder network consists of a series inductor with a shunt capacitor connected to the output side of the inductor. For higher-power designs, one or more additional LC ladder segments may be used to reduce the insertion loss of the ladder network. Both of these LC ladder networks are generally expensive due to the need for expensive high quality (Q) discrete inductors in order for the network to effectively suppress unwanted harmonics of the carrier frequency and to minimize the insertion loss of the matching network. Moreover the capacitors form low-impedance shunts at the harmonic frequencies, but the parasitics of these shunts increase current consumption at the fundamental frequency.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.